1. Field of the Invention
The present invention relates to an ultrasonic transmitter for radiating ultrasonic waves into a body of water, for instance, an ultrasonic transceiver for radiating ultrasonic waves and receiving echoes of the radiated ultrasonic waves, and a sonar apparatus including an ultrasonic transceiver for detecting objects using ultrasonic waves.
2. Description of the Related Art
Today, sonar apparatuses, such as scanning sonars, are widely used for detecting underwater objects. A scanning sonar for detecting underwater objects all around has a generally cylindrical transducer. The scanning sonar forms an ultrasonic transmitting beam oriented in all directions around the transducer by activating transducer elements arranged on a cylindrical surface of the transducer. Also, the scanning sonar forms a receiving beam oriented in a particular horizontal direction using a specific number of vertically arranged sets, or columns, of transducer elements centered on that horizontal direction. Typically, this receiving beam is rotated around the transducer to detect underwater objects in a full-circle area by successively switching the columns of transducer elements.
This kind of scanning sonar includes an ultrasonic transceiver which employs a switching-type or linear-type push-pull circuit including a transformer as a circuit for generating a driving pulse signal for activating the transducer elements. Shown in FIG. 13 is an example of a driving pulse signal generator circuit employing a switching-type circuit configuration for generating a driving pulse signal as shown in FIG. 14. Shown in FIG. 15 is an example of a driving pulse signal generator circuit employing a linear-type circuit configuration for generating a driving pulse signal.
FIG. 13 is an equivalent circuit generally showing the configuration of the switching-type driving pulse signal generator circuit, in which designated by the numeral 1 is a transducer element, designated by Tr1 and Tr2 are transistors, and designated by VB is a driving voltage of the driving pulse signal generator circuit. FIG. 14 is a time chart showing signal states in the driving pulse signal generator circuit of FIG. 13.
FIG. 15 is an equivalent circuit generally showing the configuration of the linear-type driving pulse signal generator circuit, in which designated by the numeral 1 is a transducer element, designated by the numeral 2 is a digital-to-analog (D/A) converter, designated by the numeral 3 is an amplifier, designated by the numeral 4 is an inverting amplifier, designated by Tr1 and Tr2 are transistors, and designated by VB is a driving voltage of the driving pulse signal generator circuit.
In the switching-type driving pulse signal generator circuit shown in FIG. 13, rectangular pulse signals of opposite polarities (180° shifted in phase) of a specific frequency are input into the transistors Tr1, Tr2. As the transistors Tr1, Tr2 are alternately switched on and off by the input rectangular pulse signals with specific timing, the driving pulse signal generator outputs a driving pulse signal of which waveform is shown by broken lines in FIG. 14, and this driving pulse signal is applied to the transducer element 1 across both terminals thereof. A prior art example of this kind of switching-type driving pulse signal generator circuit is described in Japanese Patent Application No. 2001-401798, for instance.
In the linear-type driving pulse signal generator circuit shown in FIG. 15, the D/A converter 2 converts an input rectangular pulse signal into an analog signal and delivers this analog signal to the amplifier 3 and the inverting amplifier 4. The amplifier 3 amplifies the input analog signal while the inverting amplifier 4 amplifies the input analog signal with a 180° phase shift (opposite polarity). As the two analog signals of opposite polarities (180° shifted in phase) are input into the transistors Tr1, Tr2, the two transistors Tr1, Tr2 are alternately switched on and off with specific timing to produce a driving pulse signal of a desired waveform, which is applied to the transducer element 1 across both terminals thereof.
The ultrasonic transceiver of the conventional scanning sonar has a pending problem to be solved as will be explained below.
Since the aforementioned switching-type driving pulse signal generator circuit produces the driving pulse signal from rectangular pulse signals of a fixed waveform, the resultant driving pulse signal also has a rectangular waveform as shown in FIG. 14.
If a transducer element is driven by the driving pulse signal having a rectangular envelope as shown in FIG. 14, an ultrasonic wave radiated from the transducer element contains not only a desired transmitting frequency component fo which is predefined but also undesired frequency components of high amplitude levels as shown in FIG. 16A.
If a scanning sonar installed on own ship transmits ultrasonic waves underwater from a transducer of which transducer elements are driven by driving pulse signals having a rectangular envelope as stated above, the transducer radiates not only the desired frequency component fo but also the undesired frequency components. On the other hand, if another ship near own ship is fitted with her own sonar apparatus which transmits and receives ultrasonic waves at a frequency f1 which is different from but relatively close to the transmitting frequency fo of the own ship's scanning sonar, the sonar apparatus on the nearby ship receives at least part of the undesired frequency components radiated from the own ship's scanning sonar. Since echo signals received by the sonar apparatus on the nearby ship are affected by the undesired frequency components radiated from the own ship's scanning sonar, the sonar apparatus on the nearby ship would present interference fringes or false images.
It is necessary to match the transducer with a transmitting beamforming circuit in impedance to transfer the driving pulse signal to the transducer with small transmission loss, and this requires a matching circuit to be inserted between the transducer and the transmitting beamforming circuit. However, frequency response (transfer function) of this kind of matching circuit often contains components (spurious) responsive to frequencies other than a center frequency. If the driving pulse signal having a rectangular envelope is transferred through this matching circuit, the spurious components of the transfer function will be superimposed on the driving pulse signal. These spurious components cause a damped oscillatory transient known as “ringing” immediately following a trailing edge of the envelope of the driving pulse signal, where a sudden change in signal level occurs, as shown in FIG. 17. When such a ringing phenomenon occurs, a single underwater object (or target) will return multiple target echoes, causing the sonar to detect false targets. In addition, the oscillatory transient caused by the ringing phenomenon following the rectangular-shaped driving pulse signal overlaps echo signals received immediately after transmission of a rectangular-shaped ultrasonic pulse signal, making it impossible to receive echoes from nearby targets, such as bottom echoes in shallow areas.
To overcome this problem, it is generally needed to reshape the rectangular-shaped driving pulse signal such that it has a gradually rising leading edge as well as a gradually falling trailing edge. It is however impossible to control the envelope shape of the driving pulse signal such that its amplitude varies gradually by the aforementioned conventional switching-type circuit configuration which produces the driving pulse signal from rectangular pulse signals of a fixed waveform. Thus, the conventional switching-type circuit configuration is associated with a problem that it can not produce a driving pulse signal of a desired waveform.
In order to control directivity of ultrasonic waves transmitted from the transducer, it is necessary to set spatial weights of the ultrasonic waves emitted from individual transducer elements. In other words, particular weights should be assigned to the amplitude of the ultrasonic waves emitted from the individual transducer elements arranged on the transducer to produce desired transmitting directivity. Again, it is impossible to vary the amplitude of the ultrasonic waves with the conventional switching-type circuit configuration which produces the driving pulse signal from rectangular pulse signals of a fixed waveform, unless the amplitude of a power supply voltage waveform is varied. Although a variable voltage power supply circuit can produce a power supply voltage waveform of a varying amplitude, the variable voltage power supply circuit uses a capacitor of a large capacity which requires a long time for charging and discharging. It is therefore impracticable to use a variable voltage power supply circuit for driving the transducer of which transducer elements must be switched at a high speed.
In contrast, the aforementioned linear-type driving pulse signal generator circuit makes it possible to shape the waveform of the driving pulse signal and vary its amplitude in an arbitrary fashion. However, it is necessary to provide D/A converters for the individual transducer elements in the linear-type circuit configuration and this results in large power consumption. Also, the linear-type circuit configuration necessitates a large number of components, resulting in an increase in overall equipment size. Furthermore, since the transistors Tr1, Tr2 are operated in their non-saturation area, there occurs a large loss, making it impossible to achieve high efficiency.